Internal combustion engines generally produce engine output torque by performing combustion in the engine cylinders. Specifically, each cylinder of the engine inducts air and fuel and combusts the air-fuel mixture, thereby increasing pressure in the cylinder to generate torque to rotate the engine crankshaft via the pistons. One method to improve engine fuel economy during deceleration is to deactivate fuel injection to all or a selected group of cylinders to thereby reduce combustion torque and increase engine braking.
The above approach can provide engine braking from engine friction and pumping work (due to manifold vacuum). The compression and expansion of air in the cylinders during the compression and expansion stroke results in energy storage and recovery, and thus may not contribute to engine braking. As such, one approach to increase engine braking is referred to as a “Jake Brake”. A Jake Brake opens the exhaust valve at top dead center of compression, thereby reducing or eliminating the energy recovery of the expansion stroke. This, in turn, can increase engine braking significantly since the unrestrained expansion is dissipating energy stored during the compression stroke.
However, since the Jake Brake essentially operates the engine as an air compressor and air pump, several issues may arise. First, since air is being pumped through the engine, emission control devices, such as three way catalysts, may be excessively cooled thereby reducing their conversion efficiency. Further, if such operation is performed during fuel-cut operation, oxygen rich exhaust gas can result in further reducing conversion efficiency due to oxidant saturation. Second, Jake Brakes may produce increased noise that can reduce customer satisfaction for passenger vehicles not familiar with Jake Brake operation.
One approach to incorporate Jake Brake type engine braking is described in U.S. Pat. No. 6,192,857, in which exhaust valve timing is adjusted to control a level of engine braking provided. See also FIG. 20 herein. However, in the approach of '857, both the intake and exhaust valve are operated when providing the engine braking, which may produce flow through the exhaust system of air during the fuel-cut operation. This can degrade catalyst performance due to cooling and saturation as noted above.
An approach to reduce airflow during fuel-cut operation is described in U.S. Pat. No. 6,526,745, in which at least one (or both) of the intake or exhaust valve is placed in a closed state to block any flow through the engine. However, while this may reduce air flow through the exhaust, engine braking effects may be lost (or significantly reduced). In other words, if there is no air flowing through the engine, engine braking due to pumping work is reduced or lost. Further, since there is no indication of any expansion or compression work being performed, engine braking may be significantly reduced.
The inventors herein have recognized the above issues. And, faced with the paradoxical approach of the prior art (where either engine braking may be obtained at the expense of catalyst performance, or catalyst performance may be maintained at the expense of engine braking), the inventors herein have developed various systems and approaches that attempt to reduce at least some of the above tradeoffs.
In one example, a method for operating at least an intake and exhaust valve in a cylinder with a piston of an engine in a vehicle may be provided. The method comprises, during conditions of net engine torque less than zero, maintaining at least one of the intake and exhaust valves in a closed position during a period, and during at least said period where said at least one valve is in said closed position: operating with the other of the intake and exhaust valve open, then closing the other of the intake and exhaust valve, and then opening the other of the intake and exhaust valve.
In this way, it may be possible to provide engine braking while reducing net flow through the engine. In other words, since one of the valves is maintained closed, flow is impeded from the intake to the exhaust, or vice versa. And, the other valve may be operated to provide expansion or compression braking in the cylinder, for example. In this way, desired engine braking can be obtained even when there is reduced braking from reduced engine pumping work.
Note that the opening of the valve can be either full or partial opening. Also note that the period can be an expressly defined period, or a variable period, for example. Further, conditions of net engine torque less than zero may be conditions where torque of the engine is actively controlled to be negative, or conditions that result in such a situation, among other conditions, for example.